This invention was partially developed with funds provided through Assistance Agreement No. 1425-97-FC-81-30006C provided b the Bureau of Reclamation, U.S. Department of the Interior. The Government may have some rights in this invention.
The present invention relates to a controlled direct drive engine system that can be-applied to pumps and compressors for moving a variety of fluids at high and low pressures and to methods for efficiently pumping fluids using a Modified Brayton cycle or the like power cycles.
Typically, such fluid-moving devices are driven by electric motors, gas turbines, steam turbines, direct-acting steam drive, or conventional Diesel or Otto cycle engines. These pumps and compressors can be of a variety of types, including: crank-driven piston or plunger, rotary screw, or centrifugal. It is generally accepted that the crank-driven pumps and compressors (positive displacement), when driven by conventional reciprocating engines, have the highest fuel-to-fluid efficiency presently available. However, these devices tend to have relatively high maintenance due to the high cycle speeds and pulsating flowxe2x80x94primarily as a result of the mechanical limitations of the crank, and conventional reciprocating engines have high emissionsxe2x80x94primarily as a result of being internal combustion. It is also generally accepted that centrifugal pumps and compressors driven by electric motors are the most widely used, which is believed to be primarily due to their smooth output flow and convenience in installation. Moreover, these centrifugal fluid-moving devices are often used even though they inherently have lower fuel-to-fluid efficiency than competing devices and even though the device must be carefully matched to the flow and pressure conditions in the system. Accordingly, solutions have been sought that would overcome these limitations and provide smooth variable flow, low cycle speeds, high fuel-to-fluid efficiency, low emissions, and the capability to efficiently match nearly any flow or pressure condition, up to the ratings of the equipment.
It is known in the art that, when a gas at high temperature and pressure is applied to a piston, the resulting force and velocity can impart energy to another piston to pump a liquid or compress a gas. One version of such a fluid-moving device is known as the direct-acting steam pump, using the Rankine thermodynamic cycle, see FIG. 1, which is shown in the Standard Handbook for Mechanical Engineers, Seventh Edition, Page 14-9. There are simplex (single piston assemblies) and duplex (double piston assemblies) versions of these pumps. However, these types of fluid-moving devices have low fuel-to-fluid efficiency, which results from the inability to expand the steam to fairly low pressures and temperatures in order to thus recover more available thermodynamic energy in the steam. This latent energy in the steam is lost when the driving cylinders are vented for the return stroke. Also, because of the thermodynamic characteristic of steam, or other vaporizing liquid that might be used in the Rankine cycle, the pressure ratio from the inlet conditions to the discharge conditions is quite large. This provides another inherent limitation of the direct-acting steam pump.
Furthermore, the direct-acting steam pump has an additional limitation because it normally operates in a xe2x80x9cbang-bangxe2x80x9d uncontrolled velocity mode. In this bang-bang mode, steam is admitted at the start of the stroke and causes the piston to travel at full velocity until it reaches the end of the strokexe2x80x94hence the term bang-bang. The inlet pressure of the energy source fluid (steam), and the resistance pressure exerted by the fluid being moved, determines the piston velocity. In addition, the piston velocity that can be achieved may often be limited by pressure drops in the pipes and valves, or it may often be necessary to throttle the steam or the fluid being moved xe2x80x94with such a pressure drop resulting in energy losses. Moreover, these losses will occur even if multiple pistons are actuated in sequence, such as in the case of a duplex configuration.
It should be apparent that what is needed is a controlled direct drive engine system to power pumps and compressors that accomplishes the functions provided by the previously mentioned devices and methods, yet overcomes the shortcomings. It is an object of the present invention to overcome these shortcomings and assure more efficient operation in systems of this type as well as addressing other needs.
In seeking to overcome these shortcomings, it was felt that modern high performance hydraulic power transmission, electronic control, and other techniques that would replace bulky cranks, gearing and housings normally associated with positive displacement energy conversion equipment might aid in finding solutions. The variable displacement capability (and high power density) of hydraulic power was felt to allow the energy conversion process to be optimized in ways that are not usually feasible with conventional positive displacement or rotodynamic energy conversion techniques, and it was felt that this capability could result in a significant improvement in energy utilization effectiveness in addition to improved efficiency. Thus, it was felt that modern hydraulic hydrostatic transmission (HST) technology should be closely examined in this respect.
Conventional hydrostatic transmission equipment includes the variable displacement, over-center, hydraulic hydrostatic pump (HSP) which includes a tiltable swash plate and which may be used to drive a fixed displacement hydraulic hydrostatic motor (HSM). Hydrostatic transmissions of this general type have gained use as variable speed transmissions replacing conventional gear transmissions. Modern agriculture equipment, such as combines and tractors, commonly use these as continuously variable transmissions. The reasons for such use include high power density and the ability to control machine speed precisely with single lever control, while maintaining efficient engine speed. The swash plate angle position of a variable displacement pump can be controlled by a variety of mechanisms, ranging from a simple manual lever control to a sophisticated electronic servo control.
It should be noted that the hydrostatic pump is a reversible device and can also function as a hydraulic motor, when pressure is applied to the ports from another source. This inherent capability further adds to the versatility of the invention, and provides capability for a variety of energy conversion purposes.
Hydrostatic transmission components are presently manufactured in sizes from 5 to 3000 horsepower. One unique characteristic of such a positive displacement hydrostatic transmission component (pump or motor) is that its best-efficiency-point (bep) is quite high over a wide range of sizes (88% to 94%), and its efficiency remains quite high over a wide range of speed, pressure, and power. By comparison, rotodynamic devices (e.g. centrifugal pumps and turbines) have only one bep; their efficiency and system effectiveness can thus dramatically decrease in comparison.
The present invention accomplishes the functions achieved by the above-mentioned prior art in a unique way, and in a preferred mode thereof, three primary energy transfer functions are accomplished. Energy is first transferred from an energy source fluid using positive displacement. A second energy transfer is then made to a piston that is driving a fluid that is being moved while a hydraulic power device controls the power being transferred to the fluid being moved. Finally, a third transfer of energy is preferably made between piston assemblies that are operating in a complementary fashion through the controlled power device. The controlled power device controls the piston velocity profile, which includes timing sequence, acceleration, velocity, deceleration, stroke distance, and dwell of the stroking pistons, in a prescribed manner. In addition, there is preferably a transfer of energy from a piston assembly having excess available power from its source to another piston assembly that may momentarily have insufficient power from its source. This energy transfer by the controlled power device allows a piston assembly having insufficient source power to still complete its stroke in accordance with its prescribed piston velocity profile while making use of what might be termed excess power that is available.
With the foregoing in mind, the present invention is directed to a controlled direct drive engine system for pumps and compressors for moving a variety of fluids at high or low pressures. Such fluids might include water, petroleum products, slurries, high viscosity fluids, natural gas, compressed air, or substantially any other liquid or gaseous fluid. Because the system is a positive displacement system, it is particularly well suited for applications where the pressure can vary over a wide range. In addition, the invention is well suited for applications where slow cycle speeds are desirable to increase the life of the component parts and reduce maintenance costs. Moreover, the engine is well suited for applications where low atmospheric emissions are important, including the reduction of emissions, such as nitrogen oxides (NOx), carbon monoxide, carbon dioxide, and other pollutants. Furthermore, it is well suited for applications where high fuel-to-fluid efficiency is particularly important in order to achieve the economical movement of fluids, and examples of such applications include the transportation of natural gas through pipelines and the movement of water for crop irrigation. Other applications where high fuel-to-fluid efficiency can be important are those associated with power generation from renewable and low intensity energy sources, such as solar energy or geothermal energy.
In accordance with one aspect of the invention, a controlled direct drive engine system is provided for solar-powered desalting of seawater using the reverse osmosis process. It should be understood that, although such a controlled direct drive engine system for reverse-osmosis applications includes certain aspects of the invention, the invention is not limited to such an application as it may be applied to a variety of other applications, including those heretofore mentioned. In general, this invention utilizes modern hydraulic power transmission and control techniques to provide a highly efficient and highly effective method for utilizing thermal energy sources to accomplish useful work in the positive displacement movement of fluids, and/or other useful work including rotary power output converted therefrom.
One preferred controlled direct drive engine embodying various features of the present invention includes the following subsystems: a heat exchange unit (HEU) for operation in conjunction with a thermal energy source, a gas displacement unit (GDU) which employs a working fluid that is used to create a reciprocating power output, a hydraulic drive unit (HDU) which includes at least one double-acting cylinder, a positive displacement fluid displacement unit (FDU), and an electronic control unit (ECU). The following describes the functions of these subsystems in the simplest form; however, it should be understood that these subsystems are not limited to the number or type of components hereinafter mentioned.
The HEU may include a thermal energy source, a heater to heat the working fluid of the engine to an elevated temperature so as to introduce thermal energy into the system, a recuperator for recovering energy within the system, and a cooler to reject thermal energy from the system -- thus establishing a desired temperature difference between the hot side and the cold side, which is a basis for all heat engines. The GDU should include expander pistons and valves and be designed to extract energy from a fluid at elevated temperature and pressure, by expanding it to a lower pressure and temperature, and produce a reciprocating power output, preferably in combination with compressor pistons and valves to elevate the pressure of a working fluid within the system. The HDU comprises controllable hydraulic power transmission devices including a cylinder and piston and is preferably designed to facilitate providing a controlled piston velocity profile for the ultimate fluid-moving piston; moreover, it should also preferably be capable of the transfer of energy between multiple piston assemblies (which form a part of the GDU) that are being operated in complementary fashion. The FDU may include pistons and valves that are arranged to appropriately move whatever fluid that is being pumped or compressed. The EDU controls the hydraulic power transmission devices of the HDU and the expander valves of the GDU in a prescribed manner to optimize the energy transfers which are accomplished within the controlled direct drive engine system. As mentioned above, the first energy transfer is from the energy source to a working fluid, the second energy transfer is from the working fluid to the fluid that is being moved, and there is preferably a third transfer of energy between piston assemblies operating in a complementary fashion through a controlled power device.
It is understood that the engine system can utilize any of the commonly known thermodynamic power cycles, including: Rankine, Stirling, Joule, simple Brayton, recuperated Brayton, and Ericsson. The Ericsson cycle is essentially the Brayton cycle with infinite intercooling and reheating, along with highly effective recuperation. The Ericsson cycle, along with the Stirling cycle, have the theoretical potential to have an efficiency equal to the Carnot cycle, which is the standard by which all other thermodynamic cycles are compared. However, there may be particular efficiency that can be obtained through the use of a Modified Brayton cycle engine, and the incorporation of such engine may be preferred. By Modified Brayton cycle engine is meant one where the working fluid (a gas or a vapor that is always above its condensation temperature) provides power output through a positive displacement cylinder and piston device, which provides efficiency and operational advantages over more commonly used turbine energy conversion devices.
The GDU and the FDU preferably include pistons that operate within sealed cylinders, with each piston set being connected to a common reciprocating piston rod or shaft. By common rod or shaft is meant a coaxial assembly of piston rods or any mechanical interconnection of such rods so that all rods move in unison. In a preferred embodiment, three piston sets are employed that operate in a complementary fashion; however, a controlled direct drive engine system might employ one or two piston sets or more than three piston sets. When multiple piston sets are used, they are preferably driven back and forth in accordance with a prescribed velocity profile, which includes sequence timing between sets, with each having similar acceleration, controlled velocity, deceleration, and dwell periods to suit a particular pumping application. Preferably, the pistons in these sets are all double-acting, with each piston dividing its respective cylinder into two working volumes so that, as a piston strokes in one direction, one of the working volumes expands while the other working volume diminishes. However, it should be understood that such a controlled direct drive engine system could also include single-acting piston sets or plungers.
It is thus a feature of the present invention to provide a controlled direct drive engine system through which one or more piston sets may be efficiently operated in a controlled manner to optimize a velocity profile of the piston set, or sets, for an intended application from a thermal energy driven, positive-displacement power output.
It is another feature of the invention to achieve high fuel-to-fluid efficiency as compared to other pumping and compression methods, or rotary power conversion from positive-displacement output.
It is still another feature of the invention to effectively use thermal energy that is generated with low emissions or thermal energy from a variety of sources, including solar, geothermal, natural gas, exhaust heat from gas turbines, or other combustion operations to efficiently pump or compress fluids, or provide rotary power conversion from positive-displacement output.
It is yet another feature of the invention to have variable flow pumping or compressing capability without additional losses due to throttling or using of separate variable speed drives.
It is a further feature of the invention to provide smooth output flow, or other prescribed flow, by controlling piston velocity profiles when driven by a Modified Brayton cycle engine or like positive-displacement power cycles.
It is a still further feature of the invention to utilize positive-displacement power output from a thermal source to efficiently pump liquids while avoiding bang-bang operation inherent in previous direct-acting pumping methods.
It is a yet further feature of the invention to provide a thermal-powered positive-displacement engine system capable of operation at low cycle speeds so as to reduce maintenance, as well as one having low accelerations in order to allow the equipment to be mounted on conventional floors without substantial mounting foundations.
One additional feature of the invention is to provide a Modified Brayton cycle engine system that can efficiently match nearly any flow or pressure condition up to the ratings of the equipment.
Another feature of the invention is to utilize positive-displacement power output from a thermal energy source to efficiently provide rotary power output to an electric generator, or similar useful rotary output purpose.